The invention relates to a system for transmitting data signals at a given symbol rate 1/T from a data transmitter to a data receiver through a channel of a transmission facility having a plurality of channels; the data transmitter comprising a data signal source connected to the said channel through a data encoder and a transmission filter; said channel being a dispersive transmission channel of a substantially minimum-phase character this transmission channel introducing into the transmitted data signal inter symbol interference as well as noise and crosstalk from similar data signals in the other channels of the transmission facility; and the data receiver comprising an equaliser of the decision feedback type which includes a feedforward filter connected between said channel and a first input to a difference circuit, a data signal regenerator connected to the output of the difference circuit and controlled by the symbol rate 1/T recovered from the transmitted data signal, and a feedback filter connected between the output of the data signal regenerator and a second input to the difference circuit.
Such a system is known from an article "Berechnung der Schrittfehlerwarscheinlichkeit bei ternarer Datenubertragung auf Teilnehmeranschluss leitungen Berucksichtigung des Nebensprechens" by H. Schenk, Frequenz, Vol. 38, No. 3, pp. 67-71, 1984, dealing with the influence of channel properties and crosstalk on the transmission of data signals through wire pairs in the existing local public telephone network.
Within the next few decades the existing analog local public telephone network will be used on a large scale for transmitting data signals and then form a constituent part of a completely digital network: the Integrated Services Digital Network (ISDN). In this ISDN a bit rate of 144 kbit/s standardized by CCITT and CEPT will be available to any subscriber for a combination of services. Together with additional information for synchronization of the receiver at a bit rate of, for example, 8 kbit/s this will lead to a data signal transmission at a bit rate of 152 kbit/s through wire pairs of the existing local public telephone network which were originally designed for transmission of analog signals having a bandwidth of approximately 4 kHz. Certainly at this relatively high bit rate, the imperfections of the existing local public telephone network such as line attenuation, crosstalk and impulse noise will result in impairments in the transmission quality.
The decision feedback equaliser (DFE) is pre-eminently suitable for combating inter symbol interference (ISI) in view of the substantially postcursive nature of ISI in an ISDN environment, which nature originates from the minimum-phase character of the wire pairs in the telephone cables of the local public telephone network. For cancelling the postcursive ISI the DFE comprises a feedback filter which synthesizes a replica of this postcursive ISI on the basis of a limited number of N symbol decisions that have already been formed and subtracts this replica from the version of the received data signal preprocessed by a feedforward filter. This feedforward filter has for its object to suppress any interference that cannot be cancelled by the feedback filter, especially the residual ISI occurring outside the span of length NT of the feedback filter, crosstalk from adjacent wire pairs and impulse noise. This feedforward filter should have a simple, preferably non-adaptive structure and yet be operative over a wide range of ambient conditions, such as varying cable lengths, varying crosstalk levels and different types of crosstalk. The amplitude level of the crosstalk suppressed by this feedforward filter should further depend as little as possible on the ever present relative phase differences between crosstalking and desired data signals. Finally, this feedforward filter should effectively suppress high-frequency disturbances, especially impulse noise. These objects are distinctly multifarious and even partly contradictary.
The complexity of this design problem has apparently led to the fact that in known transmission systems more or less ad hoc solutions for the feedforward filter are utilized; notably low-pass filters as known for linear equalisation (for example, compare the above article by Schenk). The dimensioning of such conventional feedforward filters essentially comes down to a rather delicate balance between crosstalk and residual ISI: a limitation of the filter bandwidth yields a better crosstalk suppression at the cost of additional residual ISI and vice versa. Thus, the maximum bandwidth fairly strongly depends on local ambient conditions, such as crosstalk levels which are hardly predictable in local public telephone networks and, what is more, often time-dependent. Besides, when dimensioning such feedforward filters, no account is taken of the fact that relative phase differences between crosstalking and desired data signals can significantly affect the instantaneous transmission quality.